Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, and various signal processing circuits.
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual images for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The structure of semiconductor material allows the material's electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed operations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each semiconductor die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual semiconductor die from the finished wafer and packaging the die to provide structural support, electrical interconnect, and environmental isolation. The term “semiconductor die” as used herein refers to both the singular and plural form of the words, and accordingly, can refer to both a single semiconductor device and multiple semiconductor devices.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller semiconductor die size can be achieved by improvements in the front-end process resulting in semiconductor die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
The manufacturing of smaller semiconductor devices relies on implementing improvements to horizontal and vertical electrical interconnection between multiple semiconductor devices on multiple levels, i.e., three dimensional (3-D) device integration. One approach to achieving the objectives of greater integration and smaller semiconductor devices is to embed PCB units adjacent to a semiconductor die in a single package. PCB units include preformed conductive vias, or plated through-holes (PTH), used to route electrical signals through a semiconductor package. Contact pads on a bottom, or front, side of a PCB unit are connected to an RDL formed over the PCB unit and a semiconductor die. Contact pads on a top, or back, side of the PCB unit are exposed opposite the RDL layer for subsequent interconnection with a second semiconductor package or other external device in a package on package (PoP) configuration.
Embedded PCB units used in semiconductor packages are commonly formed with contact pads on the top side of the PCB unit which are larger than contact pads on the bottom side of the PCB unit. Contact pads on the top side of a PCB unit can be formed larger due to the capability of equipment used in manufacturing the PCB unit, or because of different registration tolerances of the equipment used during subsequent interconnection steps. However, larger contact pads on the top side of a PCB unit results in more total conductive material on the top side of the PCB unit and creates an imbalance between the sides of the PCB unit. The imbalance of conductive material between the top side and bottom side of a PCB unit causes warpage in the PCB unit which proves problematic during encapsulation and compressive molding of the semiconductor package. Many common manufacturing problems which can occur during compressive molding are more likely to occur when the top side and bottom side of a PCB unit are unbalanced. Warpage of the PCB unit causes gaps between the PCB unit and a carrier. The PCB unit does not lie flat and fully contact carrier tape on the carrier when warped, leading to increased instances of mold bleed and flying PCB units.
Mold bleed occurs during compressive molding when encapsulant bleeds underneath a PCB unit. Encapsulant under the PCB unit causes manufacturing defects by covering contact pad surfaces and interfering with electrical connection between the PCB unit and a subsequently formed RDL. Flying PCB units occur when encapsulant applies a lateral force to a PCB unit during compressive molding which causes the PCB unit to move. The movement of a PCB unit during encapsulation prevents subsequent RDLs from making proper contact with the PCB unit as required by the design of the semiconductor die and package.